Electrode, use thereof, and an electrochemical cell

ABSTRACT

The invention relates to an electrode for an electrochemical cell, which is either a piece of a monocrystal grown from doped titanium dioxide or which contains a multiplicity of monocrystals grown from doped titanium dioxide.

The invention relates to an electrode for an electrochemical cell, to use thereof, and to an electrochemical cell.

In electrochemical cells, electrodes comprising a mixed oxide coating are normally used, which primarily have titanium or niobium as a substrate metal, to which noble metal oxides of the platinum group are applied with other valve metals, such as aluminum, tantalum, niobium, manganese, titanium, bismuth, antimony, zinc, cadmium, zirconium, tungsten, tin, iron silver and silicon. For chloride-free electrolytes, iridium mixed oxide coatings are normally used for the production of chlorine and hypochloride ruthenium mixed oxide coatings or iridium/ruthenium mixed oxide coatings. It is further known to use platinum-plated titanium or niobium anodes for specific applications, such as hard chrome plating, noble metal coating or the recovery of metals. Anodes of this type consist of a titanium or niobium substrate in the form of expanded metal, in rod form, in wire form, in tube form or the like, to which platinum or noble metal oxides are applied in layer thicknesses up to 20 μm in a number of processing steps. The service life of these anodes depends, in particular, on the working medium, the electrolyte, and the anode current intensity and is defined by the corrosion, in steps, of the applied layer and by the change in polarity during operation.

It is further known, in electrochemical cells, to use diamond electrodes made of doped diamond particles, which are embedded in a non-conductive substrate layer. Such diamond electrodes are characterized by a high overvoltage for oxygen and hydrogen and are therefore suitable for a multiplicity of oxidation processes in aqueous solution. A diamond electrode made of synthetically produced, electrically conductive boron-doped diamond particles is known for example from WO 2004/005585 A1. In this diamond electrode the diamond particles are embedded in the surface of a metal layer or metal alloy later. A diamond electrode in which the doped diamond particles are embedded in a non-conductive substrate layer and are exposed on both sides of the substrate layer is known from WO 2007/116014 A2. A plastics-based diamond electrode for electrochemical applications is known from WO 2006/116298 A1. This electrode, at least at its surface, has a layer made of synthetically produced doped diamond particles. The plastic main body, which is non-conductive in principle, can be made electrically conductive by admixing conductive components.

Such diamond electrodes have proven to be well suitable in practice, since they are extremely resistant and have a longer service life than the above-described metal electrodes.

The object of the invention is to provide an electrode that has a longer service life than the previously known electrodes.

The stated object is achieved in accordance with the invention in that the electrode contains a piece of a crystal grown from doped titanium dioxide or contains a multiplicity of crystals grown from doped titanium dioxide.

Electrodes that have an extremely long service life, which far exceeds the service life of the known electrodes, can be produced or provided from doped titanium dioxide in crystal form.

It is particularly advantageous that the crystal piece can be a crystal plate cut from the grown crystal. Such electrodes can be trimmed to the desired size from the grown crystal in various shapes, sizes and thicknesses, in particular in a thickness from 0.5 mm to 10 mm, preferably up to 3 mm.

With an electrode formed from a multiplicity of crystals, these crystals are embedded in a single layer in a substrate layer made of a non-conductive material, the crystals being exposed on both sides of the substrate layer. In this embodiment too, a high service life is ensured.

In particular, polytetrafluoroethylene (Teflon), polyvinylidene fluoride (PVDF), perfluoroalkoxy alkane (PFA), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene (ETFE), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polyethylene (PE), polypropylene (PP) or polyvinyl chloride (PVC) are possible materials for the substrate layer (1).

In accordance with the invention, the crystals embedded in the substrate layer have a particle size between 100 μm and 5 mm, in particular between 200 μm and 800 μm, and are grown in this size. The particle size is adapted to the thickness of the substrate layer.

In accordance with the invention, one of the following elements: lithium, niobium, aluminum, phosphorous, gallium, boron, arsenic, indium, germanium, iridium, ruthenium, rhodium, antimony, nitrogen, manganese, iron, cobalt, nickel, chromium or yttrium, or oxides or fluorides thereof is used to dope the titanium dioxide.

Electrodes according to the invention are particularly suitable for use as edge electrodes in an electrochemical cell, either as an anode or as a cathode, due to their long service life.

The use of an electrode according to the invention as a bipolar electrode in an electrochemical cell is also advantageous.

The invention further relates to an electrochemical cell that contains, as (an) edge electrode(s), an electrode or electrodes which is/are designed in accordance with the invention. The electrochemical cell may also contain at least one diamond electrode as a bipolar electrode.

In an alternative embodiment of an electrochemical cell, said cell contains, as (a) bipolar electrode(s), an electrode or a number of electrodes which is/are electrodes designed in accordance with the invention.

Further features, advantages and details of the invention will emerge from the following description and will be described in greater detail with reference to the schematic drawing, which illustrates exemplary embodiments and in which:

FIG. 1 shows a sectional illustration of a portion of an electrode according to the invention; and

FIG. 2 shows a sectional view during production of the electrode.

The invention concerns the production and design of an electrode anode or cathode for an electrochemical cell. The electrode is to have a much longer service life compared to the previously known electrodes comprising a mixed oxide coating.

In an embodiment of the invention, the electrode consists of a piece, which is plate-shaped in particular, of a monocrystal grown from doped titanium dioxide (TiO₂). To produce such electrodes, crystals of corresponding size are grown, which are cut into the desired shape, for example a rectangular or round shape, and with the desired thickness, in particular between 0.5 mm and 10 mm, preferably up to 3 mm. Doping is performed in the starting material or during crystal growing in the melt. A multiplicity of elements can be used to achieve the doping necessary to obtain the electrical conductivity, for example lithium, niobium, aluminum, phosphorous, gallium, boron, arsenic, indium, germanium, iridium, ruthenium, rhodium, antimony, nitrogen, manganese, iron, cobalt, nickel, chromium or yttrium or the oxides or fluorides of said elements. Elements or oxides/fluorides thereof that are trivalent or pentavalent may thus be used. Iridium and ruthenium are particularly suitable.

The methods known for growing monocrystals can be used to grow the titanium dioxide crystals, in particular the methods for crystal growing from a melt, such as the Bridgman-Stockbarger method. This method allows the growth of monocrystals of high quality and abundance.

In another embodiment of the invention, the electrode consists of small monocrystals 2 grown from doped titanium dioxide and embedded in a substrate material made of a non-conductive plastic.

FIG. 1 shows an embodiment of an electrode of this type, wherein the crystals 2 are embedded in a single layer in a plastic substrate layer 1 without contacting one another on either side and protrude slightly on either side from the substrate layer 1 and are exposed.

The crystals 2 may have particle sizes between approximately 100 μm to a few millimeters, in particular up to 5 mm. Particle sizes between 200 μm and 800 μm are preferred. For an intended electrode, crystals 2 having a substantially identical particle size are used, wherein the thickness of the substrate layer 1 is adapted to the particle size. The crystals 2 are doped with one of the above-mentioned doping elements, or with the oxides or fluorides thereof, and are therefore electrically conductive. The crystals 2 are grown in the desired particle size by known methods in the form of monocrystals.

In a preferred embodiment, the starting material for the substrate layer 1 is films made of chemically stable polymers, in particular polytetrafluoroethylene (Teflon), polyvinylidene fluoride (PVDF), perfluoroalkoxy alkane (PFA), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene (ETFE), polyetheretherketone (PEEK), polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC) or polyphenylene sulfide (PPS). To produce the substrate layer 1, two films 4, 5 or film webs made of the same material are used in a thickness matched to the crystal sizes. A layer of crystals 2 is applied to a horizontal or substantially horizontal film 4 positioned in a planar manner. The second film 5 is then placed onto the first film 4 provided with the crystals 2, and the two films 4, 5 are interconnected between the crystals 2. The two films 4, 5 are preferably connected with application of pressure from either side, for example by exerting pressure onto the film laminate in a press or between two rolls. If the films are also heated during this process, they melt and interconnect. If the crystals 2 on either side are already exposed as a result of the pressure applied on either side, no post-treatment is necessary. It is possible, however, to expose the crystals 2 subsequently in a mechanical, chemical or thermal manner.

So as to avoid subsequent exposure of the crystals 2, it is advantageous if all crystals 2 are already exposed at the outer faces of the films when the two films 4, 5 are joined together. In a preferred embodiment, the first film 4 is therefore placed onto a thin plate 3 of a soft, resilient material and a thin plate 3 made of this material is likewise placed onto the outer face of the second film 5, which has already been positioned, as is shown in FIG. 2. Pressure can then be applied over the surface from one or both sides and heat can be supplied, so that the films 4, 5 melt and interconnect. In doing so, the crystals 2 penetrate through the film material and are exposed. Possible preferred material for these thin, resilient plates 3 include, for example, Teflon (polytetrafluoroethylene), Viton and Kapton (fluoroelastomers by DuPont), Neoprene (chloroprene rubber (or polychloroprene or chlorobutadiene rubber), thermoplastic vulcanizates (TPV), fluoro rubbers, for example copolymers of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) and terpolymers of VDF, HFP and tetrafluoroethylene (TFE), other fluorinated elastomers, such as perfluoro rubber (FFKM), tetrafluoroethylene/propylene rubbers (FEPM) and fluorinated silicone rubber (VQM), as well as silicones, but also metals, such as lead, aluminum or copper. The thickness of the plates 3 is selected between 0.2 mm to 3 mm, in particular between 0.5 mm and 1.5 mm. If necessary, crystals 2 can be exposed subsequently in a further processing step in a mechanical, chemical or thermal manner.

To increase the mechanical strength of the finished electrode, a support lattice, support fabric 6 or the like can be positioned in one or more layers onto, or beneath, the film 4 and/or onto the applied crystals 2 during production of the electrode. As described, the two film webs are then connected to produce the substrate layer 1 and to expose the crystals 2. Alternatively it is possible, once the electrode has been fabricated, to fasten the support lattice, support fabric 6 or the like onto an outer face or to both outer faces of the electrode, for example by means of gluing or laminating. Suitable materials for the support lattice or support fabric 6 include plastics, such as polytetrafluoroethylene (Teflon), polyvinylidene fluoride (PVDF), perfluoroalkoxy alkane (PFA), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene (ETFE), polyetheretherketone (PEEK) or polyphenylene sulfide (PPS), glass fibres, plastic-coated glass fibres, ceramics or metals.

Electrodes designed or produced in accordance with the invention are particularly suitable for use in electrolysis cells (electrochemical cells), in particular for drinking water treatment, for disinfection of drinking water, for water treatment by anodic oxidation, for production of oxidizing agents and for electrolysis of water and for electrochemical production of ozone and chlorine. A preferred use is their use in electrochemical cells for purification of water in swimming pools, whirlpools or hot tubs.

Electrodes consisting of a crystal piece are particularly suitable as edge electrodes, whilst electrodes having crystals embedded in a substrate layer are particularly suitable as bipolar electrodes. An electrically conductive contacting layer can be applied on one side to an edge electrode, thus making it possible to supply the crystal with current in an optimal and lasting manner.

It is also possible, in an electrochemical cell, to combine electrodes according to the invention with electrodes from the prior art, for example to use diamond electrodes as edge electrodes or as bipolar electrodes.

LIST OF REFERENCE NUMERALS

1 . . . substrate layer

2 . . . crystal

3 . . . plate

4 . . . film

5 . . . film

6 . . . support lattice 

1. An electrode for an electrochemical cell comprising: a piece of a monocrystal grown from doped titanium dioxide; or a multiplicity of monocrystals grown from doped titanium dioxide.
 2. The electrode according to claim 1, wherein the electrode is a crystal plate cut from the grown crystal.
 3. The electrode according to claim 2, wherein the thickness of the crystal plate is between 0.5 mm and 10 mm.
 4. The electrode according to claim 1, wherein the electrode is a multiplicity of crystals embedded as a single layer in a substrate layer made of a non-conductive material, the crystals being exposed on both sides of the substrate layer.
 5. The electrode according to claim 4, wherein the crystals have a particle size between 100 μm and 5 mm, the particle size being adapted to the thickness of the substrate layer.
 6. The electrode according to claim 1, wherein the titanium dioxide is doped with lithium, niobium, aluminum, phosphorous, gallium, boron, arsenic, indium, germanium, iridium, ruthenium, rhodium, antimony, nitrogen, manganese, iron, cobalt, nickel, chromium or yttrium, or with one or more oxides or fluorides of said elements.
 7. The electrode according to claim 4, wherein the substrate layer consists of polytetrafluoroethylene (Teflon), polyvinylidene fluoride (PVDF), perfluoroalkoxy alkane (PFA), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene (ETFE), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polyethylene (PE), polypropylene (PP) or polyvinyl chloride (PVC). 8-11. (canceled)
 12. An electrochemical cell, which, as an edge electrode thereof, contains an electrode as defined in claim
 1. 13. The electrochemical cell according to claim 12, which further contains at least one diamond electrode as an edge electrode or a bipolar electrode.
 14. An electrochemical cell, which, as a bipolar electrode thereof, contains an electrode as defined in claim
 1. 15. The electrode according to claim 3, wherein the thickness of the crystal plate is up to 3 mm.
 16. The electrode according to claim 5, wherein said particle size is between 200 μm and 800 μm.
 17. The electrode according to claim 14, which further contains at least one diamond electrode as an edge electrode or a bipolar electrode. 